A few that come to mind:

- Unix/Linux fundamentals (processes/threads, builtin commands, scripting, file system/permissions, basic system admin, etc.)

- SQL

- Writing & communicating technical topics (i.e. how do you distill complex technical concepts to the appropriate level of detail to have meaningful discussions with the target audience; applies to all technical topics and becomes more important as you advance in your career)

- Testing (unit/integration/e2d; but more conceptually, how do you gain confidence that your code does what it's intended to do?)